1. Field of the Invention
The present invention relates to a screw vacuum pump, and more particularly to a screw vacuum pump suitable for an oil free type using a screw pump body having a pair of male and female screw rotors meshed with each other.
2. Description of the Prior Art
As a known vacuum pump, there exists a water sealed vacuum pump (Japanese Patent Publication No. 54-37693), a high vacuum pump (Japanese Patent Publication No. 57-59920), and a dual shaft type vacuum pump (Japanese Patent Laid-open No. 59-185889), for example.
In the water sealed vacuum pump and the high vacuum pump, gas is led from a suction opening to a discharge opening under the condition where it directly contacts with water or oil during obtaining vacuum, so as to prevent an increase in temperature of the gas due to heat generated upon compression of the gas in the pump. Thus, the device as well as the gas is cooled.
However, in this kind of vacuum pump, there is a possibility that the odor of the water or oil leaks to the suction opening. Further, in the event that the device is suddenly stopped because of a power failure, for example, the water or oil will flow reversely to the suction opening. Therefore, such a vacuum pump cannot be used in the semiconductor manufacturing industry which never permits mixing of impurities and the food industry (e.g., vacuum packaging) which particularly refuses the generation of odor.
On the other hand, the dual shaft type vacuum pump includes an oil free type such as a Root's type mechanical booster (The oil free type vacuum pump in the present invention is of a type such that the gas to be vacuumized is not in contact with the water and oil).
However, in this type of vacuum pump, a large amount of gas between the rotors is necessarily defined because both the rotors are required to be maintained without contact with each other. Accordingly, although this vacuum pump is employable in a medium vacuum range (10.sup.-3 -1 Torr), it is not suitable for use in a low vacuum range (1-760 Torr) because the leakage of gas from between the rotors increases to remarkably raise the gas temperature.
Thus, a single stage oil free type vacuum pump suitable for use over a low vacuum range to a high vacuum range has not previously been known, and it is therefore desired to develop such a vacuum pump.